Identifying groundwater vulnerability from nitrate contamination: comparison of the DRASTIC model and Environment Canterbury’s method

Abstract

Groundwater is a crucial natural resource in New Zealand, as it is used to support many environmental, economic, social and cultural aspects of the country. However, in recent years the quality of our groundwater resources has begun to degrade and is now of great concern, especially in the Canterbury region. The degradation of the groundwater has occurred over many years and is the result from a number of direct and indirect effects, but quite notably nitrogen, in the form of nitrates, used in intensive agricultural activities has been identified as one of the major contributors to this degradation. Nitrate contamination in groundwater can cause issues to the environment and to the people that rely on resource. These issues can range from increase in toxic algal blooms, to degradation of aquatic habitat, and it can cause biological effects to humans, such as blue baby syndrome. Local government agencies, such as Environment Canterbury (ECan), have now had to implement strategies and methods to monitor groundwater quality to address this situation. One of these methods that ECan has used is the development of risk maps for identifying groundwater vulnerability to nitrate contamination, which is based on water quality monitoring data. There are other methods used around the world for predicting vulnerability maps for groundwater contamination, with the most common method being the GIS-based DRASTIC model. This model produces vulnerability values that are reflected on a map, which represent the vulnerability of the groundwater from contamination, such as nitrate contamination. However, ECan has not implemented this model as a method for groundwater vulnerability. This research involves developing a DRASTIC model map for nitrate vulnerability in groundwater for the central and southern Canterbury region and comparing it with the vulnerability map produced by ECan. By doing this research, it will identify whether the DRASTIC map could be a reliable and therefore a viable monitoring tool for nitrate vulnerability in groundwater for the Canterbury region, in order to help manage Canterbury’s crucial natural groundwater resource. This dissertation presents results of a completed DRASTIC map, along with seven key areas that were identified on both the DRASTIC map and the ECan map, which were used for comparison. The comparison identified whether the vulnerability values produced on the DRASTIC map were able to predict reliable results in identifying areas of groundwater that are vulnerable to nitrate contamination based off of the nitrate concentrations represented on the ECan vulnerable map. The results showed that five of the seven key areas used comparison showed reliable results for the DRASTIC mode for the Canterbury region. In these areas the DRASTIC model was able to show that it could be used to predict areas of groundwater that are vulnerable to nitrate contamination that may potentially receive further nitrate contamination in the future. The other two key areas used for comparison showed some unreliable results, as current nitrate concentrations shown on the ECan’s maps were far greater than the vulnerability values provided by the DRASTIC model. From this it was gathered that the DRASTIC model has limitations, as the current nitrate concentrations on the ECan map were a result of current land use activities, which the DRASTIC model does not consider. Overall, from the comparison of these seven key areas, the results suggest that the DRASTIC model could be a useful monitoring tool in the future, which can be used to identify nitrate contamination in groundwater. The DRASTIC model should be used alongside other current methods that are used for monitoring groundwater contamination. The model can also then be applied to the applications of the DRASTIC model that were identified by the EPA, which will be useful for future management of the groundwater resources in the Canterbury region. Therefore it is suggested that the DRASTIC model is a reliable and a viable method as a monitoring tool for identifying areas of groundwater that are vulnerable to nitrate contamination in the Canterbury region.